The present invention generally relates to cleaning methods and equipment. More particularly, this invention relates to cleaning methods and equipment suitable for cleaning oxidized surfaces of components, for example, internal surfaces of turbine airfoil components, and to the subsequent repair of such components.
Internal cooling of components, for example, combustor liners and turbine blades (buckets) and vanes (nozzles) of gas turbine engines, is commonly employed to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. Air-cooled components of a gas turbine engine typically require that the cooling air flow is routed through a cooling circuit within the component before being discharged through carefully configured cooling holes (or slots) that distribute a cooling film over the component surface to increase the effectiveness of the cooling flow. Processes by which cooling holes are formed and configured are critical because the size, shape and surface conditions of each cooling hole opening determine the amount of air flow exiting the holes and affect the overall flow distribution within the cooling circuit containing the holes.
Air-cooled components located in the high temperature sections of gas turbine engines are typically formed of superalloys. Strenuous high temperature conditions to which these components are subjected during engine operation can lead to various types of damage or deterioration. For example, erosion, cracks and other surface discontinuities tend to develop at the tips and trailing edges of turbine blades and vanes during service due to foreign object impact (foreign object damage, or FOD). Because the material and processing costs of superalloys are relatively high, repair of damaged or worn superalloy components is typically preferred over replacement.
FIG. 1 schematically represents the blade tip region of a high pressure turbine blade 10. Deep tip cracks 12 are present in the blade tip and penetrate cooling holes 14 within the tip. Deep tip cracks of the type represented in FIG. 1 can be repaired by first mechanically routing out the crack to remove oxidized metal material and thereby yield a clean, wettable surface that can be repaired, for example, by tungsten inert gas (TIG) welding. However, mechanical routing can lead to excessive material removal, which has the effect of enlarging the crack in the parent material to the extent that molten weld material is more readily able to penetrate the wall of a component. In the case of an air-cooled turbine airfoil component such as the blade 10, the surface tension of molten weld material that has penetrated a wall can cause the weld material to form bumps within the cooling passages of the blade 10, a phenomenon sometimes referred to as drop-through. In addition to undesirably increasing the weight to the repaired blade, the drop-through weld material may constrict air flow through the cooling air circuit within the blade 10.
In view of the above, it would be advantageous if a repair process were available that was able to produce a repairable surface that conserves parent metal material, yet provides a clean, wettable surface capable of accepting a weld repair. It would be further advantageous to provide a repaired turbine blade that incurs little or no weight penalty as a result of undergoing a repair process.